[o] volute 'prev' => 'new' { ... }
[o] volute Module { ... }
[o] obj.vset(:a => 0, :b => 1)
[o] volute { over }
[o] volute Class { volute :attr { } } --> Class AND attr
    volute Class, :attr { } --> Class OR attr
[o] volute :not, Light { ... }
[o] volute /^att/ { ... }
[o] not including Volute, but triggering Volute.xxx on some changes
[o] volute :att => :not_nil { ... } (not a nil value)
[o] volute :att => [ a, b ] { ... } (a or b)
[o] volute previous_value => [ val0, val1 ] { ... } (OR)
[o] volute [ pval0, pval1 ] => value { ... } (OR)
[o] volute :att => /regex/ { ... }
[o] volute /regex/ => /regex/ { ... }
[o] rewrite README head
[o] readme : list alternatives (aspects/hooks/callbacks)

[ ] volute Invoice do
      volute :paid do
        if is(true)
          object.comment = 'got paid'
        elsif was(nil)
          object.comment = 'still not paid'
        end
      end
    end

[x] multi-entity state machine example
    not happy with the traffic light example

[ ] volutes
    `-- volute x, y
      `-- volute a, b

[ ] pass the object as block binding ?